Heat Exchanger Fluid Circuit Arrangement

ABSTRACT

An air coil unit of a heat exchanger includes tubes organized into separate circuits of serially connected tubes. The tubes of a given circuit are arranged in serially connected circuit portions, where the tubes of each circuit portion are arranged in rows. The circuits are arranged in a parallel flow-cross flow (PFCrF) configuration where the circuit crosses to adjacent rows between circuit portions to compensate for non-uniform air flow across the tubes. In the PFCrF circuit configuration, each circuit of the heat exchanger shares the non-uniformity of air flow, and mitigates the mal fluid distribution inside the tubes due to the non-uniform air flow across tubes.

BACKGROUND

Heat exchangers are used in many heating and cooling applications to control the environment within a closed space. However, the efficiency of heat exchangers may be adversely affected by non-uniform distribution of fluid flow through the device. In one example illustrated in FIG. 16, an indoor unit 40 of a conventional heat pump includes an air-to-refrigerant heat exchanger 50 disposed in a cabinet 55 along with a blower 58 configured to draw air through an air coil unit 51 of the heat exchanger 50. The air coil unit 51 is formed of several discrete fluid circuits C1, C2, . . . C8 distributed across a fluid-facing surface thereof. A filter 57 is disposed adjacent to the air coil unit 51 so as to filter air before it is drawn into the air coil unit 51 by the blower 58. In some heat pump configurations, the heat exchanger 50 functions as an evaporator in which refrigerant flowing through tubes that form the air coil unit 51 may be heated by air passing across the tubes. Due at least in part to the limited space of the cabinet 55 and the arrangement of the blower 58 and filter 57 relative to the air coil unit 51, the air flow through the air coil unit 51 is not uniformly distributed. In particular, the air velocity varies across a surface of the air coil unit 51. For example, the air velocity may be greatest in a center of the surface of the air coil unit 51, and least at a periphery of the air coil unit 51. This non-uniformity, as represented in the flow rate profile 52 illustrated at the right side of FIG. 16, results in different refrigerant flow rate (mal refrigerant distribution) in each circuit C1, C2, . . . C8 of the air coil unit 51. This is because air coil circuits that are subjected to more air flow tend to have a lower refrigerant flow rate, and the air coil circuits that are subjected to less air flow tend to have a higher refrigerant flow rate. As seen in FIG. 17, the superheating in each circuit C1, C2, . . . C8 varies depending on circuit position within the air flow profile. The disparity in refrigerant flow rate for the different circuits within the same air coil unit and resulting heat transfer capacity variation across the air coil unit reduces the heat transfer effectiveness of the air coil unit, and the overall system performance is significantly reduced over a wide range of operating conditions.

Thus, it is desirable to provide a heat exchanger in which the air coil unit is configured to compensate for non-uniform air flow across the air coil unit.

SUMMARY

In some aspects, a system for performing at least one of a heating, ventilating, air conditioning and refrigeration function is provided. The system includes a heat exchanger, the heat exchanger including an air coil unit.

In some aspects, the air coil unit of the heat exchanger includes a first tube sheet having a first surface that is parallel to the direction of air flow through the air coil unit. The air coil unit includes a second tube sheet parallel to and spaced apart from the first tube sheet. The second tube sheet has a second surface that faces the first surface and is parallel to the direction of air flow through the air coil unit. In addition, the air coil unit includes thermally conductive tubes that extend between the first surface and the second surface. The tubes are arranged so as to be transverse to the direction of air flow through the air coil unit, parallel to each other tube, spaced apart from adjacent tubes and arranged into rows. Each row defines a plane that is transverse to the direction of air flow through the air coil unit. The rows include a first row and a second row, where the second row is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit. A first subset of the tubes is serially connected to form a first circuit defining a single, continuous, unbranched first fluid path that extends between a first inlet and a first outlet. The first circuit includes a first portion of the first subset of the tubes that is disposed in the first row and a second portion of the first subset of the tubes that is disposed in the second row. A second subset of the tubes is serially connected to form a second circuit defining a single, continuous, unbranched second fluid path that extends between a second inlet and a second outlet. The second circuit includes a first portion of the second subset of the tubes that is disposed in the first row and a second portion of the second subset of the tubes that is disposed in the second row. The first inlet and the second inlet are in the first row. The first outlet and the second outlet are in a row other than the first row. In addition, the first portion of the first subset of the tubes is aligned with the second portion of the second subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.

In some embodiments, the first portion of the second subset of the tubes is aligned with the second portion of the first subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.

In some embodiments, the rows include a third row that is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, and the first circuit includes a third portion of the first subset of the tubes that is disposed in the third row and is aligned with the first portion of the first subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.

In some embodiments, the third row is disposed between the first row and the second row.

In some embodiments, the rows include a fourth row that is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, and the first circuit includes a fourth portion of the first subset of the tubes that is disposed in the fourth row and is aligned with the first portion of the second subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.

In some embodiments, the fourth row is disposed between the third row and the second row.

In some embodiments, the first outlet and the second outlet are in the second row.

In some embodiments, the heat exchanger is a fin-and-tube heat exchanger.

In some embodiments, a third subset of the tubes is serially connected to form a third circuit defining a single, continuous, unbranched third fluid path that extends between a third inlet and a third outlet, the third circuit including a first portion of the third subset of the tubes that is disposed in the first row and a second portion of the third subset of the tubes that is disposed in the second row. In addition, a fourth subset of the tubes is serially connected to form a fourth circuit defining a single, continuous, unbranched fourth fluid path that extends between a fourth inlet and a fourth outlet, the fourth circuit including a first portion of the fourth subset of the tubes that is disposed in the first row and a second portion of the fourth subset of the tubes that is disposed in the second row. The third inlet and the fourth inlet are in the first row, the third outlet and the fourth outlet are in a row other than the first row, and the first portion of the third subset of the tubes is aligned with the second portion of the fourth subset of tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.

In some embodiments, the first portion of the second subset of the tubes is disposed in the first row between the first portion of the first subset of the tubes and the first portion of the third subset of the tubes, and the first portion of the fourth subset of the tubes is disposed in the first row adjacent to the first portion of the third subset of the tubes.

A system for performing at least one of a heating, ventilating, air conditioning and refrigeration function includes a heat exchanger, the heat exchanger including fluid circuits comprised of serially-connected thermally conductive tubes. In particular, the circuits of the heat exchanger are provided in a configuration that addresses the susceptibility of air-to-refrigerant heat exchangers to non-uniform air flow distribution which contributes to mal refrigerant flow distribution among the individual circuits and hence degrades the heat transfer capacity of the heat exchanger in heat pump systems. In the heat exchanger, the circuits are arranged in a parallel flow-cross flow (PFCrF) configuration or a counter-flow-cross flow (CFCrF) configuration that compensates for non-uniform fluid flow across the tubes. In the PFCrF and CFCrF configurations, each circuit of the heat exchanger shares the non-uniformity of air flow. As a result, heat transfer efficiency is improved, particularly when the system is in a cooling mode and the air coil unit works as evaporator.

The PFCrF and CFCrF approaches described herein include providing a circuit pattern that can be repeated throughout the air coil unit. Since the circuit pattern is repeatable, overall costs of the air unit are reduced relative to some conventional approaches for eliminating or reducing the effects of non-uniform air flow distribution on the performance of the heat exchanger. For example, in some conventional approaches, the air coil is designed so that the length of each circuit is different or the tube arrangement of each circuit is customized so as to compensate and counter balance the air flow non-uniformity and have equivalent refrigerant flow rate and heat transfer capacity for each circuit. Since each circuit of the conventional approach may be different, it can be difficult to design and result in relatively high costs. Moreover, since this conventional approach results in asymmetric design of coils, the quantity of stock keeping units (sku) is increased, a concern that is compounded in an application where the same heat exchanger may have different air return configurations and/or is required to have the same performance wherever it is placed within the system.

Advantageously, the circuit patterns described herein are suitable for both forward refrigerant flow and reverse refrigerant flow applications, allowing the same heat exchanger to be used as both an evaporator and a condenser in a reversible system such as a heat pump with an optimal design for cooling and heating performance. This can be compared to some conventional heat exchanger circuits such as a cross-flow circuits, which are not able to function effectively as a condenser.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of a heat pump system.

FIG. 2 is a side view of an indoor unit of the heat pump system of FIG. 1, illustrating a heat exchanger including an air coil unit disposed in the indoor unit.

FIG. 3 is a perspective view of the air coil unit of FIG. 2.

FIG. 4 is an end view of the air coil unit of FIG. 2.

FIG. 5 is a schematic diagram of a tube arrangement of an alternative embodiment two-row air coil unit.

FIG. 6 is a schematic diagram of a tube arrangement of another alternative embodiment two-row air coil unit.

FIG. 7 is a schematic diagram of a tube arrangement of another alternative embodiment two-row air coil unit.

FIG. 8 is a schematic diagram of a tube arrangement of another alternative embodiment two-row air coil unit.

FIG. 9 is a schematic diagram of a tube arrangement of another alternative embodiment two-row air coil unit.

FIG. 10 is an end view of an alternative embodiment air coil unit having three rows.

FIG. 11 is a schematic diagram of the tube arrangement of the three-row air coil unit of FIG. 10.

FIG. 12 is a schematic diagram of a tube arrangement of an alternative embodiment air coil unit having four rows.

FIG. 13 is a schematic diagram of a tube arrangement of another alternative embodiment four-row air coil unit.

FIG. 14 is a schematic diagram of a tube arrangement of another alternative embodiment three-row air coil unit illustrating forward flow.

FIG. 15 is a schematic diagram of the tube arrangement of the three-row air coil unit of FIG. 14 illustrating counter flow.

FIG. 16 is a side view of a prior art indoor unit of a heat pump system illustrating non-uniform air flow into the heat exchanger.

FIG. 17 is a graph of superheat versus circuit number of the heat exchanger of FIG. 16 illustrating non-uniform heating within the air coil unit of the heat exchanger.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a system for performing at least one of a heating, ventilating, air conditioning and refrigeration function includes a heat exchanger 60. In one exemplary embodiment, the heat exchanger 60 may be part of a heat pump system 2 that is used to heat and cool the interior space of a building 1. The heat exchanger 60 is an air-to-refrigerant heat exchanger that includes an air coil unit 61 having fluid circuits 102, 202 comprised of serially-connected thermally conductive tubes 68. In particular, the circuits 102, 202 of the heat exchanger 60 are provided in a configuration, described below, in which each circuit 102, 202 of the air coil unit 61 shares the non-uniform distribution of air flow. As a result, the impact of non-uniform air flow including mal refrigerant flow distribution among the individual circuits and degradation of the heat transfer capacity of the heat exchanger are minimized or avoided.

The heat pump system 2 includes a fluid circuit that connects an indoor unit 4 and an outdoor unit 14 in a reversible cooling/heating loop 3 that permits the system 2 to be switchable between heating and cooling functions. To this end, each of the indoor unit 4 and the outdoor unit 14 includes a heat exchanger 16, 60 that may function either as an evaporator or a condenser depending on the heat pump operation mode. For example, when heat pump system 2 is operating in cooling mode, the heat exchanger 16 of the outdoor unit 14 functions as a condenser, releasing heat to the outside air, while the heat exchanger 60 of the indoor unit 4 functions as an evaporator, absorbing heat from the inside air. When heat pump system 2 is operating in heating mode, the heat exchanger 16 of the outdoor unit 14 functions as an evaporator, absorbing heat from the outside air, while the heat exchanger 60 of the indoor unit functions as a condenser, releasing heat to the inside air. The heat pump system 2 will be described herein as though configured to perform a cooling function within the building 1.

The reversible cooling/heating loop 3 includes the indoor unit 4 including the heat exchanger 60 functioning as an evaporator, and a blower 8 configured to draw or push air across an air coil unit 61 of the heat exchanger 60. The loop 3 also includes the outdoor unit 14. The outdoor unit includes a compressor 24, the heat exchanger 16 functioning as a condenser, and a blower 20 configured to draw or push air across an air coil unit (not shown) of the heat exchanger 16. The compressor 24 may be any suitable compressor such as a screw compressor, reciprocating compressor, rotary compressor, swing link compressor, scroll compressor, or turbine compressor. The outdoor unit 14 also includes a reversing valve 22 that is positioned in the loop 3 between the heat exchangers 16, 60 to control the direction of refrigerant flow and thereby to switch the heat pump system 2 between heating mode and cooling mode. In the illustrated example, the reversing valve 22 is controlled by a control unit 28 via, for example, a solenoid 29.

The loop 3 also includes an expander 12, 18, for example a thermal expansion valve (TXV), positioned immediately upstream of the evaporator inlet of each heat exchanger 16, 60. The TXVs 12, 18 are configured to decrease the pressure and temperature of the refrigerant before it enters the evaporator. The TXV valves 12, 18 may also regulate the refrigerant flow entering the evaporator so that the amount of refrigerant entering the evaporator equals, or approximately equals, the amount of refrigerant exiting the evaporator. Only one of the TXVs 12, 18 is used in a given operating mode of the heat pump system 2. That is, the TXV that is used is the TXV associated with the heat exchanger 16, 60 that is currently functioning as an evaporator. For example, when heat pump system 2 is operated in the cooling mode, refrigerant bypasses the TXV 18 associated with the heat exchanger 16 of the outdoor unit 14 via a first bypass line 30, and flows through the TXV 12 before entering the heat exchanger 60 of the indoor unit 4, which acts as an evaporator in the cooling mode. When heat pump system 2 is operated in the heating mode, refrigerant bypasses the TXV 12 associated with the heat exchanger 60 of the indoor unit 4 via a second bypass line 31, and flows through the TXV 18 before entering the heat exchanger 16 of the outdoor unit 14, which acts as an evaporator in the heating operating mode.

In the illustrated embodiment, the fluid that passes through the loop 3 is a refrigerant, although it is not limited thereto. The refrigerant may be any fluid that absorbs and extracts heat.

During a cooling operation, the refrigerant enters the indoor unit heat exchanger 60 (e.g., the evaporator) as a low temperature and pressure liquid. Some vapor refrigerant also may be present as a result of the expansion process that occurs in the TXV 12. The refrigerant flows through the air coil unit 61 of the heat exchanger 60 and absorbs heat from the air, changing the refrigerant into a vapor. After exiting the evaporator, the refrigerant passes through reversing valve 22 and into the compressor 24. The compressor 24 decreases the volume of the refrigerant vapor, thereby, increasing the temperature and pressure of the vapor. After exiting from the compressor 24, the increased temperature and pressure vapor refrigerant flows into the outdoor unit heat exchanger 16 (e.g., the condenser). In the heat exchanger 16, the refrigerant vapor flows into the air coil while a blower 20 draws air across the tubes of the air coil. The heat from the refrigerant is transferred to the outside air causing the refrigerant to condense into a liquid. After exiting the outdoor unit heat exchanger 16, the liquid refrigerant flows through the TXV 12 and returns to the indoor unit heat exchanger 60 (e.g., the evaporator) as a low temperature and pressure liquid, where the cooling process begins again.

A motor 26 drives the compressor 24 and circulates refrigerant through the loop 3. The operation of the motor 26 is controlled by the control unit 28. The control unit 28 receives information from an input device 34, an indoor temperature sensor 35 and an outdoor temperature sensor 36, and uses the information to control the operation of heat pump system 2 in both cooling mode and heating mode. In addition, the control unit 28 uses information received from input device 34 to switch heat pump system 2 between heating mode and cooling mode. For example, if input device 34 is set to cooling mode, the control unit 28 will send a signal to the solenoid 29 to place reversing valve 22 in an air conditioning position. Consequently, the refrigerant will flow through reversible loop 3 as described above. If the input device 34 is set to heating mode, the control unit 28 will send a signal to the solenoid 29 to place reversing valve 22 in a heating position. Consequently, the refrigerant will flow through the reversible loop 3 as follows: the refrigerant exits compressor 24, is condensed in the indoor unit heat exchanger 60, bypasses the TXV 12 via bypass 31, is expanded in the TXV 18, and is evaporated in the outdoor unit heat exchanger 16.

The control unit 28 may execute hardware or software control algorithms to regulate heat pump system 2. In some exemplary embodiments, the control unit 28 may include an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board.

Referring to FIGS. 3 and 4, the heat exchanger 60 of the indoor unit 4 is a fin-and-tube heat exchanger that includes the air coil unit 61, a fluid inlet manifold 9 that delivers refrigerant to the tubes 68 of air coil unit 61 via the distributor tubes 76, and a fluid outlet manifold 10 that receives refrigerant exiting the air coil unit 61 via distributor tubes 78. In FIG. 3, the distributor tubes 76 between the fluid inlet manifold 9 and the tubes 68 have been omitted to permit clear visualization of the tubes 68.

The air coil unit 61 includes a first tube sheet 62, a second tube sheet 66 parallel to and spaced apart from the first tube sheet 62, and thermally conductive tubes 68 that extend between an inward-facing surface 63 of the first tube sheet 62 and an inward-facing surface 67 of the second tube sheet 66. The inward-facing surfaces 63, 67 of the first and second tube sheets 62, 63 are parallel to the direction of air flow through the air coil unit 61. In the figures, the direction of air flow through the air coil unit 61 is represented by an arrow 80.

In addition, the air coil unit 61 includes thin, plate-shaped, thermally-conductive fins (not shown) that protrude outward from an outer surface of each tube 68 in a direction perpendicular to a direction of fluid flow through the tube 68 and parallel to the direction of air flow through the air coil unit 61. Each tube 68 is in physical contact with many fins, which are spaced apart along a longitudinal direction of the tube 68. For example, in some embodiments, each tube 68 has about sixteen fins per 25.4 mm of tube length. The air coil unit 61 may have plate fins or continuous fins that are shared by all the tubes 68, or each tube 68 may have fins that are separate from other tubes 68.

The tubes 68 are supported at each end by the first and second tube sheets 62, 63, and are arranged so as to be transverse to the direction of air flow 80 through the air coil unit 61. Each tube 68 of the air coil unit 61 is parallel to the other tubes 68 and spaced apart from adjacent tubes 68. The tubes 68 are arranged into rows, and each row defines a plane P that is transverse to the direction of air flow 80 through the air coil unit 61. In the embodiment illustrated in FIGS. 3-5, the air coil unit 61 includes two rows, for example, a first row R1 defining a first plane P1 and a second row R2 defining a second plane P2. In the air coil unit 61, the second row R2 is spaced apart from the first row R1 in a direction downstream with respect to the direction of air flow 80. The tubes 68 within a given row are offset relative to the tubes 68 within an adjacent row in a direction parallel to the planes P1, P2.

Along the outward-facing surfaces 64, 69 of each of the first and second tube sheets 62, 63, the ends of the tubes 68 are connected by generally U-shaped tubular joints, referred to hereafter as “U-joints” 74. The tubes 68 of the air coil unit 61 are serially connected in such a way that several separate fluid paths or “circuits” are provided. The tubes 68 within a given circuit are serially connected via the U-joints 74 to define a single, continuous, unbranched fluid path within the air coil unit 61. In the air coil unit embodiment illustrated in FIGS. 4 and 5, the tubes 68 are arranged into four circuits.

Referring to FIG. 5, which is a schematic end view of the two row, four circuit air coil unit 61 of FIGS. 3 and 4, and illustrates the interconnections between the tubes 68, an exemplary air coil circuit arrangement will now be described.

In the air coil 61, a first subset of the tubes 68 is serially connected to form a first circuit 102 that defines a single, serpentine fluid path that extends between a first circuit inlet 106 and a first circuit outlet 108. In this embodiment, the first circuit 102 includes eight tubes 68. A first portion 110 of the tubes 68 of the first circuit 102 is disposed in the first row R1. In addition a second portion 112 of the tubes 68 of the first circuit 102 is disposed in the second row R2. The first and second portions 110, 112 of the first circuit each include four serially connected tubes 68. In addition, the first portion 110 is serially connected to the second portion 112 such that fluid (i.e., refrigerant) initially flows through the first portion 110 and subsequently flows through the second portion 112.

A second subset of the tubes 68 is serially connected to form a second circuit 202 that defines a single, serpentine fluid path that extends between a second circuit inlet 206 and a second circuit outlet 208. Like the first circuit 102, the second circuit 202 includes eight tubes 68. A first portion 210 of the tubes 68 of the second circuit 202 are disposed in the first row R1. In addition a second portion 212 of the tubes 68 of the second circuit 202 are disposed in the second row R2. The first and second portions 210, 212 of the second circuit 202 each include four serially connected tubes 68. In addition, the first portion 210 is serially connected to the second portion 212 such that fluid (i.e., refrigerant) initially flows through the first portion 210 and subsequently flows through the second portion 212.

A third subset of the tubes 68 is serially connected to form a third circuit 302 that defines a single, serpentine fluid path that extends between a third circuit inlet 306 and a third circuit outlet 308. Like the first and second circuits 102, 202, the third circuit 302 includes eight tubes 68. A first portion 310 of the tubes 68 of the third circuit 302 are disposed in the first row R1. In addition a second portion 312 of the tubes 68 of the third circuit 302 are disposed in the second row R2. The first and second portions 310, 312 of the third circuit 302 each include four serially connected tubes 68. In addition, the first portion 310 is serially connected to the second portion 312 such that fluid (i.e., refrigerant) initially flows through the first portion 310 and subsequently flows through the second portion 312.

A fourth subset of the tubes 68 is serially connected to form a fourth circuit 402 that defines a single, serpentine fluid path that extends between a fourth circuit inlet 406 and a fourth circuit outlet 408. Like the other circuits 102, 202, 302, the fourth circuit 402 includes eight tubes 68. A first portion 410 of the tubes 68 of the fourth circuit 402 are disposed in the first row R1. In addition a second portion 412 of the tubes 68 of the fourth circuit 402 are disposed in the second row R2. The first and second portions 410, 412 of the fourth circuit each include four serially connected tubes 68. In addition, the first portion 410 is serially connected to the second portion 412 such that fluid (i.e., refrigerant) initially flows through the first portion 410 and subsequently flows through the second portion 412.

The circuits 102, 202, 302, 402 are arranged in the PFCrF configuration that compensates for non-uniform air flow across the tubes 68. In the PFCrF configuration, the first circuit inlet 106 and the second circuit inlet 206 are disposed in the first row R1, whereas the first circuit outlet 108 and the second circuit outlet 208 are disposed in the second row R2. In the PFCrF configuration, the first portion 110 of the first circuit 102 is aligned with the second portion 212 of the second circuit 202 when viewed in a direction parallel to the direction of air flow 80. In addition, the first and second circuits 102, 202 cross each other at a location between the first portions 110, 210 and the second portions 112, 212, and the second portion 112 of the first circuit 102 is aligned with the first portion 210 of the second circuit 202 when viewed in a direction parallel to the direction of air flow 80.

As used herein, references to circuit portion “alignment” are understood to mean the alignment of one circuit portion with another circuit portion taking into account the relative offset of tubes 68 between adjacent rows.

In the air coil unit 61, the third and fourth circuits 302, 402 are arranged in the pattern established by the first and second circuits 102, 202. In particular, the third circuit inlet 306 and the fourth circuit inlet 406 are disposed in the first row R1, whereas the third circuit outlet 308 and the fourth circuit outlet 408 are disposed in the second row R2. In addition, the first portion 310 of the third circuit 302 is aligned with the second portion 412 of the fourth circuit 402 when viewed in a direction parallel to the direction of air flow 80, the third and fourth circuits 302, 402 cross each other at a location between the first portions 310, 410 and the second portions 312, 412, and the second portion 312 of the third circuit 302 is aligned with the first portion 410 of the fourth circuit 402 when viewed in a direction parallel to the direction of air flow 80.

In the air coil unit 61 illustrated in FIGS. 3-5, the first circuit inlet 106, the second circuit inlet 206, the third circuit inlet 306 and the fourth circuit inlet 406 are each disposed in the first row R1, which is the outermost row that faces air flow coming into the air coil unit 61. In addition, the first circuit outlet 108, the second circuit outlet 208, the third circuit outlet 308 and the fourth circuit outlet 408 are each disposed in the second row R2, which is downstream relative to the first row R1 with respect to the direction of air flow 80, and is the outermost row on the air outlet side of the air coil 61.

The air coil unit 61 described above with respect to FIGS. 3-5 includes four circuits 102, 202, 302, 402. In some embodiments, however, the air coil may include more than four circuits, as discussed in the following example.

Referring to FIG. 6, an alternative embodiment air coil 261 includes six circuits 2102, 2202, 2302, 2402, 2502, 2602. The first, second, third and fourth circuits 2102, 2202, 2302, 2402 of the air coil 261 are identical to the first, second, third and fourth circuits 102, 202, 302, 402 of the air coil 61 illustrated in FIG. 5 except that each circuit 2102, 2202, 2302, 2402 includes six tubes 68, and each circuit portion has three tubes 68. Thus the description of the first through fourth circuits is not repeated and common reference numbers are used to refer to common elements.

In the air coil 261, a fifth subset of the tubes 68 is serially connected to form the fifth circuit 2502 that defines a single, serpentine fluid path that extends between a fifth circuit inlet 506 and a fifth circuit outlet 508. The fifth circuit 2502 includes six tubes 68. A first portion 510 of the tubes 68 of the fifth circuit 2502 is disposed in the first row R1. In addition a second portion 512 of the tubes 68 of the fifth circuit 2502 is disposed in the second row R2. The first and second portions 510, 512 of the fifth circuit 2502 each include three serially connected tubes 68. In addition, the first portion 510 is serially connected to the second portion 512 such that fluid (i.e., refrigerant) initially flows through the first portion 510 and subsequently flows through the second portion 512.

A sixth subset of the tubes 68 is serially connected to form a sixth circuit 2602 that defines a single, serpentine fluid path that extends between a second circuit inlet 606 and a second circuit outlet 608. Like the other circuits of this embodiment, the sixth circuit 2602 includes six tubes 68. A first portion 610 of the tubes 68 of the sixth circuit 2602 are disposed in the first row R1. In addition a second portion 612 of the tubes 68 of the sixth circuit 2602 are disposed in the second row R2. The first and second portions 610, 612 of the sixth circuit 2602 each include three serially connected tubes 68. In addition, the first portion 610 is serially connected to the second portion 612 such that fluid (i.e., refrigerant) initially flows through the first portion 610 and subsequently flows through the second portion 612.

The circuits 2102, 2202, 2302, 2402 are arranged in the PFCrF configuration described above with respect to FIG. 5. The fifth circuit inlet 506 and the sixth circuit inlet 606 are disposed in the first row R1, whereas the fifth circuit outlet 508 and the sixth circuit outlet 608 are disposed in the second row R2. In addition, the first portion 510 of the fifth circuit 2502 is aligned with the second portion 612 of the sixth circuit 2602 when viewed in a direction parallel to the direction of air flow 80. In addition, the fifth and sixth circuits 2502, 2602 cross each other at a location between the first portions 510, 610 and the second portions 512, 612, and the second portion 512 of the fifth circuit 2502 is aligned with the first portion 610 of the sixth circuit 2602 when viewed in a direction parallel to the direction of air flow 80.

In the air coil unit 261, the first circuit inlet 106, the second circuit inlet 206, the third circuit inlet 306, the fourth circuit inlet 406, the fifth circuit inlet 506 and the sixth circuit inlet 606 are each disposed in the first row R1, which is the outermost row that faces air flow coming into the air coil unit 261. In addition, the first circuit outlet 108, the second circuit outlet 208, the third circuit outlet 308, the fourth circuit outlet 408, the fifth, circuit outlet 508 and the sixth circuit outlet 608 are each disposed in the second row R2, which is downstream relative to the first row R1 with respect to the direction of air flow 80 through the air coil unit 261, and is the outermost row on the air outlet side of the air coil 261.

The air coil units 61, 261 described above with respect to FIGS. 4-6 each include the same number of tubes in each circuit portion. For example, in the air coil unit 61 illustrated in FIG. 5, four tubes are provided for each of the first circuit portion 110 and the second circuit portion 112 of the first circuit 102, the first circuit portion 210 and the second circuit portion 212 of the second circuit 202, the first circuit portion 310 and the second circuit portion 312 of the third circuit 302, and the first circuit portion 410 and the second circuit portion 412 of the fourth circuit 102. In some embodiments, however, the air coil unit may include circuits in which portions within the same circuit have different numbers of tubes 68, as discussed in the following example.

Referring to FIG. 7, an alternative embodiment air coil 361 includes a first circuit 3102, a second circuit 3202, a third circuit 3302 and a fourth circuit 3402. The first, second, third and fourth circuits 3102, 3202, 3302, 3402 of the air coil 361 are similar to the first, second, third and fourth circuits 102, 202, 302, 402 of the air coil 61 illustrated in FIG. 5, including being arranged in the PFCrF configuration. However, the first, second, third and fourth circuits 3102, 3202, 3302, 3402 of the air coil 361 differ from the circuits 102, 202, 302, 402 of the air coil 61 illustrated in FIG. 5 with respect to the number of tubes per circuit, and the number of tubes per circuit portion. In particular, in the air coil unit 361, each circuit 3102, 3202, 3302, 3402 includes 10 tubes 68, and each circuit portion within a circuit does not have the same number of tubes. In the illustrated embodiment, the first circuit 3102 includes a first circuit portion 3110 having six tubes 68 and a second circuit portion 3112 having four tubes 68, the second circuit 3202 includes a first circuit portion 3210 having four tubes 68 and a second circuit portion 3212 having six tubes 68, the third circuit 3302 includes a first circuit portion 3310 having four tubes 68 and a second circuit portion 3312 having six tubes 68, and the fourth circuit 3402 includes a first circuit portion 3410 having six tubes 68 and a second circuit portion 3412 having four tubes 68.

The air coil units 61, 261, 361 described above with respect to FIGS. 4-7 each include an even number of circuits. For example, the air coil unit 61 illustrated in FIG. 5 includes four circuits, and the air coil unit 261 illustrated in FIG. 6 includes six circuits. In some embodiments, however, the air coil unit may include an odd number of circuits, as discussed in the following two examples.

Referring to FIG. 8, an alternative embodiment air coil 461 has three circuits including a first circuit 4102, a second circuit 4202, and a third circuit 4302. The second and third circuits 4202, 4302 are similar to the third and the fourth circuits 3302, 3402 described above with respect to FIG. 7, including having ten tubes 68 each and being arranged in the PFCrF configuration. However, first circuit 4102 of the air coil 461 differs from the circuits previously described. In particular, the first circuit 4102 does not pair with another circuit in a crossed configuration, and instead is a parallel flow circuit having 12 tubes. The first circuit 4102 includes a first circuit inlet 4106 disposed in the first row R1 and a first circuit outlet 4108 disposed in the second row R2. The first circuit 4102 includes a first circuit portion 4110 and a second circuit portion 4112. The first circuit portion 4110 has six tubes, is disposed in the first row R1 and is serially connected to a second circuit portion 4112. The second circuit portion 4112 has six tubes, and is disposed in the second row R2 so as to be aligned with the first circuit portion 4110 with respect to the direction of air flow 80 through the air coil unit 461.

The first circuit portion 4310 of the third circuit 4302 is disposed in the first row R1 between the first circuit portion 4110 of the first circuit 4102 and the first circuit portion 4210 of the second circuit 4202. In addition, the second circuit portion 4212 of the second circuit 4202 is disposed in the second row R2 between the second circuit portion 4112 of the first circuit 4102 and the second circuit portion 4312 of the third circuit 4302.

Referring to FIG. 9, an alternative embodiment air coil 561 has five circuits including a first circuit 5102, a second circuit 5202, a third circuit 5302, a fourth circuit 5402 and a fifth circuit 5502. The first, second, third and fourth circuits 5102, 5202, 5303, 5402 are identical to the first, second, third and fourth circuits 102, 202, 302, 402 described above with respect to FIG. 5, including being arranged in the PFCrF configuration. Thus, common elements are referred to with common reference numbers.

In addition, the fifth circuit 5502 of the air coil 561 is similar to the first circuit 4102 described above with respect to FIG. 8. However, the fifth circuit 5502 of the air coil 561 differs from the first circuit 4102 described above with respect to FIG. 8 with respect to the number of tubes per circuit, and the number of tubes per circuit portion. The fifth circuit 5502 includes eight tubes 68 and four tubes 68 per circuit portion.

A first circuit portion 5510 of the fifth circuit 5502 is disposed in the first row R1 between the first circuit portion 210 of the second circuit 5102 and the first circuit portion 310 of the third circuit 5302. In addition, the second circuit portion 5212 of the fifth circuit 5502 is disposed in the second row R2 between the second circuit portion 112 of the first circuit 5102 and the second circuit portion 412 of the fourth circuit 5402.

The air coil units 61, 261, 361, 461, 561 described above with respect to FIGS. 4-9 each include two rows R1, R2. In some embodiments, however, the air coil unit may include more than two rows, as discussed in the following examples.

Referring to FIGS. 10 and 11, an alternative embodiment air coil 661 includes a first row R1 defining a first plane P1, a second row R2 defining a second plane P2, and a third row R3 defining a third plane P3. The planes P1, P2, P3 are transverse to the direction of air flow 80 through the air coil unit 661. The second row R2 and the third row R3 are spaced apart from the first row R1 in a direction downstream with respect to the direction of air flow 80, and the third row R3 is disposed between the first row R1 and the second row R2. The first row R1 is the outermost row of the air coil 661 on the air inlet side of the air coil 661 and the second row R2 is the outermost row of the air coil 661 on the air outlet side of the air coil 661.

In the air coil 661, a first subset of the tubes 68 is serially connected to form a first circuit 6102 that defines a single, serpentine fluid path that extends between a first circuit inlet 6106 and a first circuit outlet 6108. In this embodiment, the first circuit 6102 includes twelve tubes 68. A first portion 6110 of the tubes 68 of the first circuit 6102 is disposed in the first row R1, a second portion 6112 of the tubes 68 of the first circuit 6102 is disposed in the second row R2, and a third portion 6114 of the tubes 68 of the first circuit 6102 are disposed in the third row R3. The first, second and third portions 6110, 6112, 6114 of the first circuit 6102 each include four serially connected tubes 68. In addition, the first portion 6110 is serially connected to the second portion 6112 such that fluid (i.e., refrigerant) initially flows through the first portion 6110, then flows through the third portion 6114 and then finally flows through the second portion 6112.

A second subset of the tubes 68 is serially connected to form a second circuit 6202 that defines a single, serpentine fluid path that extends between a second circuit inlet 6206 and a second circuit outlet 6208. In this embodiment, the second circuit 6202 includes twelve tubes 68. A first portion 6210 of the tubes 68 of the second circuit 6202 is disposed in the first row R1, a second portion 6212 of the tubes 68 of the second circuit 6202 is disposed in the second row R2, and a third portion 6214 of the tubes 68 of the second circuit 6202 are disposed in the third row R3. The first, second and third portions 6210, 6212, 6214 of the second circuit 6202 each include four serially connected tubes 68. In addition, the first portion 6210 is serially connected to the second portion 6212 such that fluid (i.e., refrigerant) initially flows through the first portion 6210, then flows through the third portion 6214 and then finally flows through the second portion 6212.

A third subset of the tubes 68 is serially connected to form a third circuit 6302 that defines a single, serpentine fluid path that extends between a third circuit inlet 6306 and a third circuit outlet 6308. In this embodiment, the third circuit 6302 includes twelve tubes 68. A first portion 6310 of the tubes 68 of the third circuit 6302 is disposed in the first row R1, a second portion 6312 of the tubes 68 of the third circuit 6302 is disposed in the second row R2, and a third portion 6314 of the tubes 68 of the third circuit 6302 are disposed in the third row R3. The first, second and third portions 6310, 6312, 6314 of the third circuit 6102 each include four serially connected tubes 68. In addition, the first portion 6310 is serially connected to the second portion 6312 such that fluid (i.e., refrigerant) initially flows through the first portion 6310, then flows through the third portion 6314 and then finally flows through the second portion 6312.

A fourth subset of the tubes 68 is serially connected to form a fourth circuit 6402 that defines a single, serpentine fluid path that extends between a fourth circuit inlet 6406 and a fourth circuit outlet 6408. In this embodiment, the fourth circuit 6402 includes twelve tubes 68. A first portion 6410 of the tubes 68 of the fourth circuit 6402 is disposed in the first row R1, a second portion 6412 of the tubes 68 of the fourth circuit 6402 is disposed in the second row R2, and a third portion 6414 of the tubes 68 of the fourth circuit 6102 are disposed in the third row R3. The first, second and third portions 6410, 6412, 6414 of the fourth circuit 6402 each include four serially connected tubes 68. In addition, the first portion 6410 is serially connected to the second portion 6412 such that fluid (i.e., refrigerant) initially flows through the first portion 6410, then flows through the third portion 6414 and then finally flows through the second portion 6412. The third circuit 6302 and the fourth circuit 6402 cross each other between the respective first portions 6310, 6410 and the respective third portions 6314, 6414.

A fifth subset of the tubes 68 is serially connected to form a fifth circuit 6502 that defines a single, serpentine fluid path that extends between a fifth circuit inlet 6506 and a fifth circuit outlet 6508. In this embodiment, the fifth circuit 6502 includes twelve tubes 68. A first portion 6510 of the tubes 68 of the fifth circuit 6502 is disposed in the first row R1, a second portion 6512 of the tubes 68 of the fifth circuit 6502 is disposed in the second row R2, and a third portion 6514 of the tubes 68 of the fifth circuit 6502 are disposed in the third row R3. The first, second and third portions 6510, 6512, 6514 of the fifth circuit 6502 each include four serially connected tubes 68. In addition, the first portion 6510 is serially connected to the second portion 6512 such that fluid (i.e., refrigerant) initially flows through the first portion 6510, then flows through the third portion 6514 and then finally flows through the second portion 6512.

A sixth subset of the tubes 68 is serially connected to form a sixth circuit 6602 that defines a single, serpentine fluid path that extends between a sixth circuit inlet 6606 and a sixth circuit outlet 6608. In this embodiment, the sixth circuit 6602 includes twelve tubes 68. A first portion 6610 of the tubes 68 of the sixth circuit 6602 is disposed in the first row R1, a second portion 6612 of the tubes 68 of the sixth circuit 6602 is disposed in the second row R2, and a third portion 6614 of the tubes 68 of the sixth circuit 6602 are disposed in the third row R3. The first, second and third portions 6610, 6612, 6614 of the sixth circuit 6602 each include four serially connected tubes 68. In addition, the first portion 6610 is serially connected to the second portion 6612 such that fluid (i.e., refrigerant) initially flows through the first portion 6610, then flows through the third portion 6614 and then finally flows through the second portion 6612. The fifth circuit 6502 and the sixth circuit 6602 cross each other between the respective first portions 6510, 6610 and the respective third portions 6514, 6614.

The circuits 6102, 6202, 6302, 6402, 6502, 6602 are arranged in the PFCrF configuration that compensates for non-uniform air flow across the tubes 68. In particular, the inlets 6106, 6206, 6306, 6406, 6506, 6606 of each circuit 6102, 6202, 6302, 6402, 6502, 6602 are disposed in the first row R1, whereas the outlets 6108, 6208, 6308, 6408, 6508 of each circuit 6102, 6202, 6302, 6402, 6502, 6602 are disposed in the second row R2.

The first portion 6110 of the first circuit 6102 is aligned with the second and third portions 6212, 6214 of the second circuit 6202 when viewed in a direction parallel to the direction of air flow 80. The first circuit 6102 and the second circuit 6202 cross each other between the respective first portions 6110, 6210 and the respective third portions 6114, 6214. In addition, the second and third portions 6112, 6114 of the first circuit 6102 are aligned with the first portion 6210 of the second circuit 6202 when viewed in a direction parallel to the direction of air flow 80.

The third and fourth circuits 6302, 6402 are arranged in a manner similar to the first and second circuits 6102, 6202. In particular, the first portion 6310 of the third circuit 6302 is aligned with the second and third portions 6412, 6414 of the fourth circuit 6402 when viewed in a direction parallel to the direction of air flow 80. The third circuit 6302 and the fourth circuit 6402 cross each other between the respective first portions 6310, 6410 and the respective third portions 6314, 6414. In addition, the second and third portions 6312, 6314 of the third circuit 6302 are aligned with the first portion 6410 of the fourth circuit 6402 when viewed in a direction parallel to the direction of air flow 80.

The fifth and sixth circuits 6302, 6402 are also arranged in a manner similar to the first and second circuits 6102, 6202. In particular, the first portion 6510 of the fifth circuit 6502 is aligned with the second and third portions 6612, 6614 of the sixth circuit 6602 when viewed in a direction parallel to the direction of air flow 80. The fifth circuit 6502 and the sixth circuit 6602 cross each other between the respective first portions 6510, 6610 and the respective third portions 6514, 6614. In addition, the second and third portions 6512, 6514 of the fifth circuit 6502 are aligned with the first portion 6610 of the sixth circuit 6602 when viewed in a direction parallel to the direction of air flow 80.

Referring to FIG. 12, an alternative embodiment air coil 761 includes a first row R1 defining a first plane P1, a second row R2 defining a second plane P2, a third row R3 defining a third plane P3, and a fourth row R4 defining a fourth plane P4. The planes P1, P2, P3, P4 are transverse to the direction of air flow 80 through the air coil unit 661. The second row R2, the third row R3 and the fourth row R4 are spaced apart from the first row R1 in a direction downstream with respect to the direction of air flow 80. In addition, the third row R3 is disposed between the first row R1 and the second row R2, and the fourth row R4 is disposed between the third row R3 and the second row R2. The first row R1 is the outermost row of the air coil 661 on the air inlet side of the air coil 661 and the second row R2 is the outermost row of the air coil 661 on the air outlet side of the air coil 661.

In the air coil 761, a first subset of the tubes 68 is serially connected to form a first circuit 7102 that defines a single, serpentine fluid path that extends between a first circuit inlet 7106 and a first circuit outlet 7108. In this embodiment, the first circuit 7102 includes 32 tubes 68. A first portion 7110 of the tubes 68 of the first circuit 7102 is disposed in the first row R1, a second portion 7112 of the tubes 68 of the first circuit 7102 is disposed in the second row R2, a third portion 7114 of the tubes 68 of the first circuit 7102 is disposed in the third row R3, and a fourth portion 7116 of the tubes 68 of the first circuit 7102 is disposed in the fourth row R4. The first, second, third and fourth portions 7110, 7112, 7114, 7116 of the first circuit 7102 each include eight serially connected tubes 68. In addition, the first portion 7110 is serially connected to the second portion 7112 such that fluid (i.e., refrigerant) initially flows through the first portion 7110, then flows through the third portion 7114 and then the fourth portion 7116, and then finally flows through the second portion 7112.

A second subset of the tubes 68 is serially connected to form a second circuit 7202 that defines a single, serpentine fluid path that extends between a second circuit inlet 7206 and a second circuit outlet 7208. In this embodiment, the second circuit 7202 includes 32 tubes 68. A first portion 7210 of the tubes 68 of the second circuit 7202 is disposed in the first row R1, a second portion 7212 of the tubes 68 of the second circuit 7202 is disposed in the second row R2, a third portion 7214 of the tubes 68 of the second circuit 7202 are disposed in the third row R3, and a fourth portion 7216 of the tubes 68 of the second circuit 7202 are disposed in the fourth row R4. The first, second, third and fourth portions 7210, 7212, 7214, 7216 of the second circuit 7202 each include eight serially connected tubes 68. In addition, the first portion 7210 is serially connected to the second portion 7212 such that fluid (i.e., refrigerant) initially flows through the first portion 7210, then flows through the third portion 7214 and then the fourth portion 7216 and then finally flows through the second portion 7212.

The circuits 7102, 7202 are arranged in the PFCrF configuration that compensates for non-uniform air flow across the tubes 68. In particular, the inlets 7106, 7206 of each circuit 7102, 7202 are disposed in the first row R1, whereas the outlets 7108, 7208 of each circuit 7102, 7202 are disposed in the second row R2. The first and third portions 7110, 7114 of the first circuit 7102 are aligned with the second and fourth portions 7212, 7216 of the second circuit 7202 when viewed in a direction parallel to the direction of air flow 80. The first circuit 7102 and the second circuit 7202 cross each other between the respective third portions 7114, 7214 and the respective fourth portions 7116, 7216. In addition, the second and fourth portions 7112, 7116 of the first circuit 7102 are aligned with the first and third portions 7210, 7214 of the second circuit 7202 when viewed in a direction parallel to the direction of air flow 80.

Referring to FIG. 13, an alternative embodiment four-row air coil 861 is similar to the embodiment illustrated in FIG. 12. However, the air coil 861 illustrated in FIG. 13 differs from the air coil 761 illustrated in FIG. 12 with respect to the arrangement of the manifolds 9, 10 and corresponding inlet and outlet distributor tubes 76, 78. For example, in the air coil 861 illustrated in FIG. 13, the inlet distributor tubes 76 are disposed at opposed ends of the first row R1, and the outlet distributor tubes 78 are disposed at opposed ends of the second row R2. As a result of this configuration, the inlet and outlet manifolds 9, 10 of the air coil 861 are relatively longer, and thus more expensive to manufacture, than the inlet and outlet manifolds 9, 10 of the air coil 761 illustrated in FIG. 12. This is because, in the configuration illustrated in FIG. 12, the inlet distributor tubes 76 are disposed adjacent each other in a midportion of the first row R1, and the outlet distributor tubes 78 are disposed adjacent each other in a midportion of the second row R2.

In addition to the effect on manifold size, the arrangement of inlet and outlet distributor tubes 76, 78 illustrated in FIG. 13 results in a different refrigerant flow pattern within the air coil 861 as compared to the embodiment illustrated in FIG. 12. In particular, in the air coil 861, a first subset of the tubes 68 is serially connected to form a first circuit 8102 that defines a single, serpentine fluid path that extends between a first circuit inlet 8106 and a first circuit outlet 8108. In this embodiment, the first circuit 8102 includes 32 tubes 68. A first portion 8110 of the tubes 68 of the first circuit 8102 is disposed in the first row R1, a second portion 8112 of the tubes 68 of the first circuit 8102 is disposed in the second row R2, a third portion 8114 of the tubes 68 of the first circuit 8102 is disposed in the third row R3, and a fourth portion 8116 of the tubes 68 of the first circuit 8102 is disposed in the fourth row R4. The first, second, third and fourth portions 8110, 8112, 8114, 8116 of the first circuit 8102 each include eight serially connected tubes 68. In addition, the first portion 8110 is serially connected to the second portion 8112 such that fluid (i.e., refrigerant) initially flows through the first portion 8110, then flows through the third portion 8114 and then the fourth portion 8116, and then finally flows through the second portion 8112.

A second subset of the tubes 68 is serially connected to form a second circuit 8202 that defines a single, serpentine fluid path that extends between a second circuit inlet 8206 and a second circuit outlet 8208. In this embodiment, the second circuit 8202 includes 32 tubes 68. A first portion 8210 of the tubes 68 of the second circuit 8202 is disposed in the first row R1, a second portion 8212 of the tubes 68 of the second circuit 8202 is disposed in the second row R2, a third portion 8214 of the tubes 68 of the second circuit 8202 are disposed in the third row R3, and a fourth portion 8216 of the tubes 68 of the second circuit 8202 are disposed in the fourth row R4. The first, second, third and fourth portions 8210, 8212, 8214, 8216 of the second circuit 8202 each include eight serially connected tubes 68. In addition, the first portion 8210 is serially connected to the second portion 8212 such that fluid (i.e., refrigerant) initially flows through the first portion 8210, then flows through the third portion 8214 and then the fourth portion 8216 and then finally flows through the second portion 8212.

The circuits 8102, 8202 are arranged in the PFCrF configuration that compensates for non-uniform air flow across the tubes 68. In particular, the inlets 8106, 8206 of each circuit 8102, 8202 are disposed in the first row R1, whereas the outlets 8108, 8208 of each circuit 8102, 8202 are disposed in the second row R2.

The first and second portions 8110, 8112 of the first circuit 2102 are aligned with the third and fourth portions 8214, 8216 of the second circuit 7202 when viewed in a direction parallel to the direction of air flow 80. The first circuit 8102 and the second circuit 8202 cross each other at two locations within the air coil 861. The first cross occurs between the respective first portions 8110, 8210 and the respective third portions 8114, 8214. The second cross occurs between the respective fourth portions 8116, 8216 and the respective second portions 8112, 8212. In addition, the third and fourth portions 8114, 8116 of the first circuit 8102 are aligned with the first and second portions 8210, 7212 of the second circuit 8202 when viewed in a direction parallel to the direction of air flow 80.

Referring to FIGS. 1, 14 and 15, the heat pump system 2 includes the reversible cooling/heating loop 3 that permits the system 2 to be switchable between heating and cooling functions. As previously mentioned, the heat exchanger 60 functions either as an evaporator or a condenser depending on the heat pump operation mode. For example, when heat pump system 2 is operating in cooling mode, refrigerant is pumped through the loop 3 in a first direction by the compressor 24 such that the refrigerant enters the air coil unit, for example the alternative embodiment air coil unit 961 illustrated in FIG. 14. The air coil unit 961 has three rows R1, R2, R3 of tubes and includes six circuits 9102, 9202, 9302, 9402, 9502, 9602, each circuit 9102, 9202, 9302, 9402, 9502, 9602 having three circuit portions of four tubes 68 each. In the air coil unit 961, refrigerant enters the first row R1, travels through the tubes 68 of the first portion of each circuit, moves to the second row R2, travels through the tubes 68 of the second portion of each circuit, moves to the third row R3 and travels through the tubes 68 of the third portion of each circuit before exiting the air coil unit 961. Thus, the refrigerant passing through the tubes 68 from Row 1 to Row 2 is moving in the same general direction as the warm air passing over the tubes 68, e.g., in the direction of air flow 80. This type of flow, in which the refrigerant and the air travel in the same direction through the air coil 961, is referred to as “forward flow.” However, the air coil unit 961 and previously described air coil units 61, 261, 361, 461, 561, 661, 761, 861, are also capable of operating in reverse or counter flow. In a reverse flow configuration, such as occurs when the heat exchanger 60 of the indoor unit operates in heating mode, the refrigerant flows in a direction opposite to that of the forced airflow F. For example, when heat pump system 2 is operating in heating mode, refrigerant is pumped through the loop 3 in a second, opposite direction by the compressor 24 such that the refrigerant enters air coil 961 via the second row R2, travels through the tubes 68 of the second portion of each circuit, moves to the third row R3, travels through the tubes 68 of the third portion of each circuit, moves to the first row R1 and travels through the tubes 68 of the first portion of each circuit before exiting the air coil unit 961 (FIG. 15). Reverse flow operation is advantageous since it may maintain a more uniform temperature difference between the refrigerant and the forced airflow throughout the whole extension of the air coil unit 961, thus obtaining a better performance from the heat exchanger 60.

The heat exchangers described herein were discussed with respect to use in the heat pump system 2 that is used to control the interior environment of a building and is switchable between heating and cooling functions. It is contemplated that the heat exchangers are not limited to reversible heat pump applications, but can be used in non-reversible air conditioning, heating, or ventilation units, and can be used in other heat exchange applications such as refrigerators, vehicle radiators, etc.

Although the expanders 12, 18 within the cooling loop 3 are described herein as being thermal expansion valves (TXV), they are not limited to being a TXV. For example, the expanders may alternatively be orifices or capillary tubes.

In the illustrated embodiment, the blower 20 draws air across the tubes of the outdoor unit heat exchanger 16. However, the heat pump system 2 is not limited to this configuration. For example, in some embodiments, the blower 20 may be replaced by a pump that draws fluid across the tubes of the outdoor unit heat exchanger 16.

The air coil units described herein include tubes 68 arranged such that tubes 68 of one row are offset with respect to the tubes 68 of an adjacent row when view in the direction of air flow 80. However, it is understood that the air coil units may include tubes 68 that are arranged “in-line” such that tubes of one row are aligned with tubes 68 of an adjacent row when viewed in the direction of air flow 80.

Selective illustrative embodiments of the system including an air coil unit of a heat exchanger are described above in some detail. It should be understood that only structures considered necessary for clarifying the system, the heat exchanger and air coil unit have been described herein. Other conventional structures, and those of ancillary and auxiliary components of the system, the heat exchanger and air coil unit, are assumed to be known and understood by those skilled in the art. Moreover, while a working example of the system, the heat exchanger and air coil unit have been described above, the system, the heat exchanger and air coil unit are not limited to the working examples described above, but various design alterations may be carried out without departing from the system, the heat exchanger and air coil unit as set forth in the claims. 

What is claimed, is:
 1. An air coil unit of a heat exchanger, the air coil unit comprising a first tube sheet having a first surface that is parallel to the direction of air flow through the air coil unit; a second tube sheet parallel to and spaced apart from the first tube sheet, the second tube sheet having a second surface that faces the first surface and is parallel to the direction of air flow through the air coil unit; and thermally conductive tubes that extend between the first surface and the second surface, the tubes arranged so as to be transverse to the direction of air flow through the air coil unit, parallel to each other tube, spaced apart from adjacent tubes, arranged into rows, each row defining a plane that is transverse to the direction of air flow through the air coil unit, the rows including a first row and a second row, where the second row is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, wherein a first subset of the tubes is serially connected to form a first circuit defining a single, continuous, unbranched first fluid path that extends between a first inlet and a first outlet, the first circuit including a first portion of the first subset of the tubes that is disposed in the first row and a second portion of the first subset of the tubes that is disposed in the second row, a second subset of the tubes is serially connected to form a second circuit defining a single, continuous, unbranched second fluid path that extends between a second inlet and a second outlet, the second circuit including a first portion of the second subset of the tubes that is disposed in the first row and a second portion of the second subset of the tubes that is disposed in the second row, the first inlet and the second inlet are in the first row, the first outlet and the second outlet are in a row other than the first row, and the first portion of the first subset of the tubes is aligned with the second portion of the second subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 2. The air coil unit of claim 1, wherein the first portion of the second subset of the tubes is aligned with the second portion of the first subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 3. The air coil unit of claim 1, wherein the rows include a third row that is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, and the first circuit includes a third portion of the first subset of the tubes that is disposed in the third row and is aligned with the first portion of the first subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 4. The air coil unit of claim 3, wherein the third row is disposed between the first row and the second row.
 5. The air coil unit of claim 3, wherein the rows include a fourth row that is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, and the first circuit includes a fourth portion of the first subset of the tubes that is disposed in the fourth row and is aligned with the first portion of the second subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 6. The air coil unit of claim 5, wherein the fourth row is disposed between the third row and the second row.
 7. The air coil unit of claim 1, wherein the first outlet and the second outlet are in the second row.
 8. The air coil unit of claim 1, wherein the heat exchanger is a fin-and-tube heat exchanger.
 9. The air coil unit of claim 1, wherein a third subset of the tubes is serially connected to form a third circuit defining, a single, continuous, unbranched third fluid path that extends between a third inlet and a third outlet, the third circuit including a first portion of the third subset of the tubes that is disposed in the first row and a second portion of the third subset of the tubes that is disposed in the second row, a fourth subset of the tubes is serially connected to form a fourth circuit defining a single, continuous, unbranched fourth fluid path that extends between a fourth inlet and a fourth outlet, the fourth circuit including a first portion of the fourth subset of the tubes that is disposed in the first row and a second portion of the fourth subset of the tubes that is disposed in the second row, the third inlet and the fourth inlet are in the first row, the third outlet and the fourth outlet are in a row other than the first row, and the first portion of the third subset of the tubes is aligned with the second portion of the fourth subset of tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 10. The air coil unit of claim 9, wherein the first portion of the second subset of the tubes is disposed in the first row between the first portion of the first subset of the tubes and the first portion of the third subset of the tubes, and the first portion of the fourth subset of the tubes is disposed in the first row adjacent to the first portion of the third subset of the tubes.
 11. A system for performing at least one of a heating, ventilating, air conditioning and refrigeration function, the system comprising a heat exchanger, the heat exchanger including an air coil unit having a first tube sheet having a first surface that is parallel to the direction of air flow through the air coil unit; a second tube sheet parallel to and spaced apart from the first tube sheet, the second tube sheet having a second surface that faces the first surface and is parallel to the direction of air flow through the air coil unit; and thermally conductive tubes that extend between the first surface and the second surface, the tubes arranged so as to be transverse to the direction of air flow through the air coil unit, parallel to each other tube, spaced apart from adjacent tubes, arranged into rows, each row defining a plane that is transverse to the direction of air flow through the air coil unit, the rows including a first row and a second row, where the second row is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, wherein a first subset of the tubes is serially connected to form a first circuit defining a single, continuous, unbranched first fluid path that extends between a first inlet and a first outlet, the first circuit including a first portion of the first subset of the tubes that is disposed in the first row and a second portion of the first subset of the tubes that is disposed in the second row, a second subset of the tubes is serially connected to form a second circuit defining a single, continuous, unbranched second fluid path that extends between a second inlet and a second outlet, the second circuit including a first portion of the second subset of the tubes that is disposed in the first row and a second portion of the second subset of the tubes that is disposed in the second row, the first inlet and the second inlet are in the first row, the first outlet and the second outlet are in a row other than the first row, the first portion of the first subset of the tubes is aligned with the second portion of the second subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 12. The system of claim 11, wherein the first portion of the second subset of the tubes is aligned with the second portion of the first subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 13. The system of claim 11, wherein the rows include a third row that is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, and the first circuit includes a third portion of the first subset of the tubes that is disposed in the third row and is aligned with the first portion of the first subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 14. The system of claim 13, wherein the third row is disposed between the first row and the second row.
 15. The system of claim 13, wherein the rows include a fourth row that is spaced apart from the first row in a direction downstream with respect to the direction of air flow through the air coil unit, and the first circuit includes a fourth portion of the first subset of the tubes that is disposed in the fourth row and is aligned with the first portion of the second subset of the tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 16. The system of claim 15, wherein the fourth row is disposed between the third row and the second row.
 17. The system of claim 11, wherein the first outlet and the second outlet are in the second row.
 18. The system of claim 11, wherein the heat exchanger is a fin-and-tube heat exchanger.
 19. The system of claim 11, wherein a third subset of the tubes is serially connected to form a third circuit defining a single, continuous, unbranched third fluid path that extends between a third inlet and a third outlet, the third circuit including a first portion of the third subset of the tubes that is disposed in the first row and a second portion of the third subset of the tubes that is disposed in the second row, a fourth subset of the tubes is serially connected to form a fourth circuit defining a single, continuous, unbranched fourth fluid path that extends between a fourth inlet and a fourth outlet, the fourth circuit including a first portion of the fourth subset of the tubes that is disposed in the first row and a second portion of the fourth subset of the tubes that is disposed in the second row, the third inlet and the fourth inlet are in the first row, the third outlet and the fourth outlet are in a row other than the first row, and the first portion of the third subset of the tubes is aligned with the second portion of the fourth subset of tubes when viewed in a direction parallel to the direction of air flow through the air coil unit.
 20. The system of claim 19, wherein the first portion of the second subset of the tubes is disposed in the first row between the first portion of the first subset of the tubes and the first portion of the thud subset of the tubes, and the first portion of the fourth subset of the tubes is disposed in the first row adjacent to the first portion of the third subset of the tubes. 